The present invention relates to electron gun assemblies, and particularly to self-indexing pressed multiform support rods for such gun assemblies.
The electrostatic lens elements of an electron gun assembly are serially arranged to accelerate and focus at least one electron beam along a generally longitudinally-extending electron beam path. The lens elements of the gun assembly are mechanically secured to at least a pair of generally longitudinally-extending insulating support rods by means of support tabs extending from the lens elements and embedded into the support rods. The support rods are formed by pressing a glass powder in a mold. The support rods are subsequently fired to strengthen the rods, to fix the dimension of the rods and to remove volatile matter from the pressed support rods.
The support tabs may be integral with the lens element or the support tabs may be attached, for example by welding, to the body of the lens elements. In either case, the portions of the support tabs embedded into the support rods include shaped projections or claws formed into the end of the support tabs to firmly anchor the tabs within the support rods. Attachment of the tabs to the support rods is accomplished in an operation called beading. Occasionally, during the beading operation, one or more of the support rods become misaligned resulting in improper spacing between lens elements or resulting in incomplete coverage of the claw of the support tab by the insulating support rod. Either condition is undesirable and causes distortion of the electrostatic fields within the electron gun assembly which perturb the electron beam.
A typical apparatus utilized to fabricate an electron gun structure of a pickup tube is shown in FIG. 8 of U.S. Pat. No. 4,169,239 issued to Ehata et al. on Sept. 25, 1979. In FIG. 8 of the Ehata et al. patent, the insulating support rods are supported on beading bases which are rotated toward the stacked lens elements. As the Ehata et al. patent discloses, if the viscosity of the fused glass support rod is low, the accuracy with which the electrodes are assembled is decreased due to thermal and mechanical shock created at the time of molten supporting rod contacts the lens element support tabs.
It is known in the art that a secure but somewhat random placement of the insulating support rod on the beading base can be accomplished by providing the beading base with a vacuum holding capability. However, because of interrelated width tolerances between the support rod and the beading base, the support rod can be offset in a lateral direction during initial placement on the beading base.
An example of a structure for reducing the lateral movement of the support rod is shown in U.S. Pat. No. 3,609,400 issued to Marks et al. on Sept. 28, 1971. In the Marks et al. structure, the beading block includes a beading trough in which the insulating support rod is nested. The accuracy of the support rod alignment in the Marks et al. patent depends on the accuracy with which the width of the support rod can be controlled. The present industrial width tolerance for pressed multiform support rods up to 49 mm in length is .+-.0.254 mm. A secondary machining operation, after firing of the bead to outgas the bead and set its physical dimensions, is time consuming, expensive and thus impractical. It therefore is desirable to design a self-indexing insulating support rod that is substantially independent of the industrial width tolerance described above.